This invention relates to a vacuum type brake booster for a vehicle. More particularly, the present invention relates to a vacuum type brake booster for a vehicle that is capable of compensating for shortages in the brake pedal depressing force during emergency braking operation.
There have been results of analysis that oftentimes, when a driver rapidly depresses the brake pedal because an obstruction has suddenly appeared in the running path of the vehicle, the stepping force which the driver applies to the brake pedal is relatively small as compared with the stepping force required to produce a brake fluid pressure of a magnitude leading to a wheel lock, so that the braking capability of the vehicle is not fully exerted. On the basis of these results, it has been proposed to install on the vehicle a device for determining, from the brake pedal depression speed and the rate of increase of oil pressure in a master cylinder, whether the braking operation is a normal braking operation or an emergency braking operation, and when it is judged that the braking operation is the emergency braking operation, the brake fluid pressure is automatically raised up to a magnitude resulting in the wheel lock.
Various constructions have been proposed for such the devices, and for example one of these devices is described in Japanese Laid Open Patent Publication No. H11-48947 (1999). This discloses a vacuum type brake booster that is constructed in such a manner that its input-output characteristics can be switched by means of a control signal from outside between at least two different characteristics (a characteristic for normal braking and a characteristic for emergency braking). With the input-output characteristics of the vacuum type brake booster being switched from the characteristic for normal braking to the characteristic for emergency braking at times of emergency braking operation, it is possible to obtain the same result as when the driver depresses the brake pedal strongly.
However, this conventional vacuum type brake booster includes a solenoid for switching its characteristics from the normal braking operation to the characteristics for emergency braking, and a valve seat member driven by the solenoid, apart from an annular seat valve for atmosphere inflow control. The solenoid driving signal is outputted from an electronic control means for processing signals from a pedal stroke sensor and a master cylinder oil pressure sensor. The solenoid is driven on the basis of such the signals in a way that the valve seat member having the annular valve seat for the atmosphere inflow control is away from an opposed seal member and a valve mechanism is in an output increase state. This brake booster is considerably expensive because of a necessity of the solenoid, several kinds of the sensors, the electronic control means and the like.
Therefore, it is a primary object to provide a vacuum type brake booster capable of obtaining an output increase state during the emergency braking operation by a simple and cheap mechanical structure.
To achieve the object, there is provided a vacuum type brake booster comprising: a housing for defining at least one pressure chamber therein; a movable wall member disposed in the housing to divide the pressure chamber into a constant pressure chamber and a variable pressure chamber and to be movable in the forward and backward directions with respect to the housing; a power piston connected to the movable wall member; an input member disposed in the power piston to be movable in the forward and backward directions with respect to the power piston; an output member for outputting a propulsion force of the power piston in response to the movement of the movable wall outside the booster; an annular valve seat for atmospheric air outflow control disposed in the power piston to be directed rearwardly; a valve mechanism having a movable portion opposed to the annular valve seat for atmospheric air outflow control and a fixed portion secured air-tightly to the power piston; the annular valve seat for atmospheric air outflow control being secured to a valve seat member which is movable in the forward and backward directions with respect to the power piston; further, a holding means for holding the valve seat member at a predetermined front position in the case that an advanced distance of the input member with respect to the power piston is less than a predetermined value; a movable means for shifting the valve seat member up to a predetermined distance at a rear position in the case that an advanced distance of the input member with respect to the power piston is more than the predetermined value; and a returning means for returning the valve seat member at the predetermined front position in the case that the power piston is in a predetermined position with respect to the housing.
According to this booster, in the case that a relative movement of the input member and the power piston is larger than the predetermined value, i.e., in the cases of emergency braking operation wherein the driver hurriedly depresses the brake pedal, an engagement of the valve seat member by the holding means is released and the valve seat member is moved rearward up to the predetermined distance by use of the movable means, so that the valve seat member presses the movable member of the valve mechanism rearward. As the result, the variable pressure chamber is rapidly and forcibly communicated with an atmosphere and a propulsion or thrust force more than the thrust force obtained during the normal braking operation is applied to the movable wall member, the power piston and in its turn the output member to produce a sufficiently large magnitude of the oil pressure for braking. In other words, by changing a jumping characteristic (when the braking operation is performed by depressing the brake pedal, a ratio of the output force relative to the input force becomes an infinity until the input member is abutted on the reaction member, this is referred to as the jumping), the thrust force larger than the thrust force as obtained during the normal braking operation is applied to the output member. Namely, the vacuum type brake booster having two kinds of input-output characteristics for the normal braking characteristic and the emergency braking characteristic is provided by the simple structure.
Further, the wording of xe2x80x9crearxe2x80x9d indicates the brake pedal side with respect to the vacuum type brake booster or the vehicle""s rear side and,the wording of xe2x80x9cfrontxe2x80x9d indicates the master cylinder side or the vehicle""s front side.
Preferably, the holding means is located at a portion of the front inner-peripheral side of the power piston and has a holding member or holding members of which a front end is engaged with the power piston and a rear end is engaged with the valve seat member. According to this structure, since the valve seat member is engaged with the holding member, which is engaged with the power piston, the valve seat member is securely engaged with the power piston.
More preferably, one of the input member and the holding means is provided with a tapered portion and the holding state of the holding means is released by a cam action of the cam portion and the input member or the holding means, when an advanced distance of the input member with respect to the power piston is more than the predetermined value. It is possible to rapidly shift the braking operation from the characteristic for normal braking to the characteristic for emergency braking.
The annular valve seat for atmosphere outflow control is composed of a first annular valve seat for atmosphere outflow control which is integral with the power piston and a second annular valve seat for atmosphere outflow control which is aligned substantially with the first annular valve seat and movable forward and rearward with respect to the power piston. According to this structure, by separating the annular valve seat for atmosphere outflow control from the first annular valve seat for atmosphere outflow control which is integral with the power piston and the second annular valve seat which is aligned substantially with the first annular valve seat for atmosphere outflow control, the vacuum type brake booster having two kinds of input-output characteristics for the normal braking characteristic and the emergency braking characteristic is constructed only by adding the second annular valve seat for atmosphere outflow control to the power piston of the conventional brake booster having the annular valve seat for atmosphere outflow control. This leads to a low price of the brake booster due to the share of parts as used in the conventional brake booster.
The additional effects and objects of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings. Further, the term of xe2x80x9cfrontxe2x80x9d as used in the description indicates the front side of the vehicle and the term of xe2x80x9crearxe2x80x9d as used in the description indicates the rear side of the vehicle.